1. Field of the Invention
This invention relates to an optical attenuator, and more particularly to a voltage-controlled variable optical attenuator used in fiber-optic transmission systems, having a deformable mirror structure. This invention also relates to techniques for fabricating such an optical attenuator.
2. Description of Related Art
In a fiber-optic transmission system, it is often necessary to couple optically one or more optical fibers with one or more other optical fibers. Further, it is also often necessary to reduce the power level of an optical signal to be received by the receiving optical fiber. The device used to achieve such result is often known as an optical attenuator. For example, an optical attenuator is used to match the intensity of the incoming optical signal to the optimum operating level of a receiver. An optical attenuator is also an essential element of any gain equalization component in a fiber-optic transmission system, which aims at readjusting the power in the optical channel to correct signal distortions experienced throughout the transmission spans.
One of the common ways of making an optical attenuator is to employ a movable mirror, upon which the incident optical signal will impinge. In the nominal position, the mirror is initially adjusted so that there is minimum attenuation of the incident optical signal. An external force will then be applied to rotate the mirror, causing the reflected optical signal to deviate from its initial direction. The deviation, often controlled through the magnitude of the external force, will effect a partial misalignment of the reflected beam relative to the axis of the receiving optical fiber. The misalignment will then result in a corresponding attenuation of the input optical signal at the receiving end.
There are several general design considerations for an optical attenuator with a rotatable mirror. The first is the reliability of the actuating mechanism, as well as the magnitude of the actuating force required. It is generally desirable to maintain a low level of actuating force, thus less power consumption, required to move the mirror. The second is the size of the mirror and the entire attenuator as a whole. With the prevailing trend toward miniaturization of optical components, it is desirable to reduce the size of the attenuator while maintaining its reliability. Finally, modern optical components arc often highly priced. One of the reasons is the complicated manufacturing processes that are involved in making miniaturized, reliable components. Thus, another desirable characteristic of an optical attenuator is its ability to be mass-produced in a simple and cost efficient manner. There exist some known proposals for the design of an optical attenuator or a rotatable or deformable mirror; however, all of them have shortcomings in light of the design considerations described above.
U.S. Pat. No. 5,915,063 to Colbourne, et. al. describes a variable optical attenuator that has a flexure member consisting of a bridge portion joining two prongs, which move flexibly relative to the bridge portion. A mirror is mounted on the bridge portion. It proposes an actuating system, whereby the two prongs will change their dimensions in response to a control signal, which can be heat, electric field, or magnetic field. The asymmetric expansion or contraction of the two prongs will then cause the bridge portion, and thus the mirror, to tilt. However, the inventors recognize certain shortcomings in the proposed actuating system. For example, if piezoelectric elements are used for the two prongs, while they have a short response time to the voltage applied across them, then they are subject to hysteresis, which impairs the reliability and repetitiveness of the operation of the attenuator. Further, the electrode materials tend to migrate into the piezoelectric materials as a result of sustained voltage, causing potential short-circuiting in the piezoelectric members. When other materials, such as metals, are used to respond to thermal signals, the response time is considerably longer.
U.S. Pat. No. 5,022,745 to Zaykowski, et. al. describes an electrostatically deformable single crystal dielectrically coated mirror comprising a highly conductive thick substrate layer and a highly conductive thinner membrane layer separated from the thick layer by an insulator. The insulator is etched at its center to form a cavity. A voltage is applied between the membrane and the thick substrate to cause the membrane to deform. The inventors suggest that the deformable mirror can be used in tunable optical filters and steeling laser beams. The ""745 patent discloses both the thick and thinner membrane layers to be highly doped conducting silicon wafers. For the thinner membrane layer, one surface of the thinner membrane, facing away from the thick layer, must be polished optically flat before a dielectric coating is applied on it to form a mirror.
Further, the inventors emphasize that the membrane must be of a certain thickness, enough to support the high-quality, multilayer, dielectric coatings required for many optical applications. The inventors specifically limit some of their claims to a mirror layer with a thinner substrate, which is substantially thicker than 10 microns and is sufficiently thick to support the mirror layer.
In an article titled xe2x80x9cMicromachined Adapative Mirrorsxe2x80x9d by Gleb Vdovin of the Laboratory of Electronic Instrumentation at Delft University of Technology in the Netherlands, found on the Internet through the link, http://guernsey.et.tudelft.nl/tyson4/index.html, the author discloses a bulk micromachined adaptive mirror that consists of a thin membrane made of silicon nitride. The author also discloses the use of electrostatic control as an actuating mechanism to deform a mirror. However, in order to increase the sensitivity of the mirror to the electrostatic voltage applied, and thus increasing the deformation range of the mirror, a biasing voltage is applied to the flexible membrane of the mirror. This system increases the number of electrodes that needs to be in place, thus complicating the manufacturing processes. Further, the author suggests using a high bias voltage in the range of 100 to 300 V, together with lower control voltages in the range of 10 to 50 V. The use of a high bias voltage also decreases the practicality, flexibility and usability of this attenuation system in a fiber-optic transmission network.
This present invention provides a simple and reliable optical attenuator, which has an extremely thin membrane for the mirror and can be operated at low voltage. Further, by using photolithography and bulk micromachining technology, this invention provides an optical attenuator that is sensitive to the changes of the low actuating voltage and yet can still be manufactured in a simple and cost efficient manner.
This invention provides a very sensitive optical attenuator, which can be used to couple and attenuate optical signals between optical fibers, with a wide range of attenuation level. Such an optical attenuator includes a flexible conductive membrane to be moved by an external force, such as electrostatic force, to achieve deformation of the conductive membrane. The conductive membrane can be formed, for example, by a low pressure chemical vapor deposited silicon nitride film. A thin metallic, conductive layer is then deposited on the flexible membrane to form a reflective mirror to receive and reflect incident optical signals. The semiconductor structure includes one or more spacing posts with which the first structural member is to be joined and bonded. Electrodes are placed on the semiconductor structure in close proximity to the flexible membrane. At various areas of the semiconductor structure, additional spacing posts are added to cause deformation of the conductive membrane when a voltage is applied between the membrane and the electrodes on the semiconductor structure. The spacing between the membrane and the semiconductor structure is determined by the height of the spacer pads. The height of the spacer pads can be chosen based on various parameters, including the optical properties of the optical signals to be attenuated by such attenuator, as the amount of deformation allowed for the membrane will affect the wavefront qualities as well as the mode field of the reflected beam. In one embodiment, the precise height of such spacer pads is determined by a controlled epitaxial growth of silicon crystal over the semiconductor structure.
In accordance with a particular embodiment of the invention, in addition to the flexible membrane and semiconductor structure disclosed above, such an optical attenuator also includes an original light source emitting an optical signal via a fiber optic waveguide and a receiving fiber optic waveguide to receive reflected optical signal. In one embodiment, a dual collimator is placed over the flexible membrane so that the incident optical signal is emitted through one collimator and reflected by the conductive membrane, and the reflected signal is received through the second collimator. The dual collimator is chosen so that the angle at which the incident optical signal is emitted from it is a relatively constant and known value. The dual collimator is positioned so that the incident optical signal is directed to areas of the conductive membrane in the vicinity of a strategically located spacing post. When a voltage is applied between the conductive membrane and the electrodes on the second structural member, marked deformation of the conductive membrane occurs near the area of the strategically located spacing post. The incident optical signals are then received by the conductive membrane at or near the deformed areas and reflected at an angle that corresponds to the degree of deformation, which in turn is dependent on the voltage applied. Further, the degree of deformation, controllable by means of the applied voltage, will also affect the mode field and wavefront quality of the reflected optical signal, which together with the angle of deviation of the reflected optical signal can determine the ultimate level of attenuation achieved by such an optical attenuator.
Since the flexible membrane is micromachined to a very thin dimension, the actuating voltage required to deform such membrane is very low. Also because of the negligible mass of the flexible membrane, the device has very high shock tolerance. Also, since there are no moving parts that may cause wear due to friction, an optical attenuator of this invention is very reliable. Further, the addition of the support pads on the semiconductor structure is a cost efficient way to cause controlled deformation of the membrane so as to attain a wide attenuation range as well as a desired level of attenuation.
The present invention will be more fully understood in light of the following detailed description taken together with the drawings.